This project is designed to determine the role of peritubular concentrations of specific solutes on reabsorption of tubular fluid by the proximal tubule of the mammalian kidney. The basis for these studies is previous information obtained in our laboratory showing that the peritubular concentration of certain solutes is different in different regions of the capillary circulation in the renal cortex, and differs significantly from those in the general systemic circulation. These changes may be important in determining and controlling the functions of the proximal tubule. Derangements in disease states could potentially manifest through changes in peritubular environment in ways not previously considered.